Preheat the oven to 350 degrees F. Generously grease a 9 count mini loaf pan or other mini bar cookie pan.In a large mixing bowl, stir together the softened butter and both sugars. Stir in the eggs and vanilla.
Add the salt and baking soda and stir until well mixed, then gradually stir in the flour.
Add the chocolate chips and M&Ms and stir until mixed. Using a generously rounded tablespoon (equal to a little over 2), divide the batter evenly among the loaf indentations.
Spread the batter out with the back of the spoon. It doesn’t seem to matter if it goes all the way to the edges of the pan, as the dough spreads as it bakes.
Bake the cookie bars for 15 to 18 minutes or until they appear set, edges are brown and a toothpick inserted comes out clean.
Let cool in the pan for about 10 minutes, then loosen edges with a butter knife and invert.